It is "above the level of the model orography". So principially it is AGL, but be aware of ECMWF model resolution which is 9km, i.e. terrain is sampled with 9km step. So if you are in mountainous terrain, you can not rely on it, do not use it for planning in such cases.
More about Cloud bases: Can you tell us what do you use it for? Are you a pilot? We are thinking of adding new ECMWF parameter called Ceiling, which is similar to present Cloud base parameter. It may be interesting for you. If you want to know more, you can read this short description: https://software.ecmwf.int/wiki/display/FCST/43r1+new+parameters%3A+Ceiling
What do you think about this parameter? Would it be more useful for you than Cloud base?

I think ceiling would be useful from pilots perspective. Currently I check cloud base and low cloud to get an idea of the base and amount of cloud. Ceiling seems to combine both these with the >50% filter. I think I would use ceiling first and then maybe base and low cloud if I needed more info.

@stevenmogg, @meteo-GR We switched to ECMWF parameter Ceiling, because we found it more usefull.
In Windy it has the same name (cloud base), but now it shows height of clouds where the cloud cover is >50%.